We checked uploading it through WinSCP client (5.5.2) directly not through .NET code; this time the file uploaded successfully on Root without any error message. That means it is not a server problem.
Please attach a session log file showing that.
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Bug reports without an attached log file are usually useless.
We checked uploading it through WinSCP client (5.5.2) directly not through .NET code; this time the file uploaded successfully on Root without any error message. That means it is not a server problem.